Block copolymers, i.e., polymers comprised of segments of two or more polymers linked to one another, can be synthesized from two or more base monomers in order to obtain a polymer having physical properties which are different from those of the constituent polymers. For example, a hard, brittle polymer with a high tensile strength can be copolymerized with a soft, elastomeric polymer having low glass transition temperature (T.sub.g) and low modulus in order to obtain a block copolymer having hard and soft segments which is both elastomeric and has a high tensile strength. Such block copolymers can be more suitable for uses, such as plastic bags and packaging films, than homopolymers synthesized from the constituent polymers.
U.S. Pat. No. 5,342,395 by Jarrett et al., issued Aug. 30, 1994, and assigned to American Cyanamid Company discloses a method for producing block copolymers of lactide and .epsilon.-caprolactone, which consist of up to about 20 mole % .epsilon.-caprolactone. Jarrett et al. disclose that .epsilon.-caprolactone can be polymerized in the presence of diethylene glycol and stannous octoate in a nitrogen purged stirred reactor maintained at 200.degree. C. until maximum viscosity is achieved, after which time lactide is added to the reactor in order to produce the block copolymer. The entire process for producing the block copolymer purportedly takes between 3 and 4 hours, after which the products are ground and dried under a vacuum for an additional 12 to 18 hours at 100.degree. C. and 0.2 mm Hg to remove the unreacted monomers.
There are, however, a number of disadvantages to known processes for producing block copolymers. Most processes are commercially or economically unacceptable because of the length of time it takes to complete the block copolymerization reaction. Also, the relatively high polymerization temperatures used in known processes, i.e., temperatures up to and in excess of 200.degree. C., can lead to color formation in the block copolymer produced due to the prolonged reaction time.
Accordingly, it would be desirable to provide fast and efficient methods for producing environmentally degradable block copolymers which are commercially and economically acceptable. It would be desirable to provide methods for producing environmentally degradable block copolymers which do not result in excessive color formation in the block copolymers produced.